1. Field of the Invention
This invention relates to a catadioptric lens system, and more particularly to a catadioptric lens system capable of an automatic focusing operation.
2. Description of the Prior Art
A catadioptric lens system capable of an automatic focusing operation has not yet been developed so far. In particular, conventional automatic focusing cameras employ a so-called phase difference detecting method for a focus detection operation thereof. According to the phase difference detecting method, a focus detecting operation is performed using two axial beams of light which pass near the optical axis of the lens on an exit pupil of the lens. (Here, an axial beam of light denotes a beam of light from an object on the optical axis of a lens in a field for photographing.) However, in the case of a catadioptric lens system, which includes at least a first mirror (hereinafter referred to as "main mirror") disposed to reflect light coming from an object toward an object side and a second mirror (hereinafter referred to as "secondary mirror") disposed to reflect the light from the main mirror toward an image side, beams of light for such focus detection are just eclipsed due to the presence of a secondary mirror. Accordingly, detection of a focus condition and automatic focusing operation cannot be achieved with a camera which employs a catadioptric lens system.
However, where a camera effects focus detecting operation using beams of light which pass through positions on the exit pupil at which no eclipse is caused by the secondary mirror as distinct from such an automatic focusing camera as described above, the focus detecting operation and the automatic focusing operation can be achieved with the camera using a catadioptric lens system. Such automatic focusing camera is disclosed, for example, in U.S. Pat. No. 4,794,416.
By the way, in an automatic focusing camera of the so-called body driven type wherein a focusing lens unit is driven by a motor provided in a camera body to effect focusing, the driving force of the motor is transmitted to the focusing lens unit when operative connection is established between a lens side coupler and a camera body side coupler which is connected to the motor in the camera body. Meanwhile, the lens unit includes a speed reducing system for transmitting a driving force received by the coupler on the lens side to a focusing lens set of the lens unit at a reduced speed. Such automatic focusing camera of the body driven type is disclosed, for example, in U.S. Pat. No. 4,449,807 and U.S. Pat. No. 4,553,817.
In a catadioptric lens system, however, if it is attempted to disposed a speed reducing system for transmission of driving force from a camera body side at a location nearer to an object than a primary mirror, then either the outer diameter of a lens barrel must be increased or a projected portion must be provided on an outer periphery of a lens barrel because beams of light at such location nearer to the object than the primary mirror pass at comparatively far positions from the optical axis.
A similar problem arises with a catadioptric lens unit used for an automatic focusing camera of the so-called in-lens motor type wherein a motor of a specific configuration such as an arcuate configuration is disposed in a lens unit for directly driving a focusing lens set. In particular, if it is attempted to dispose such motor at a location nearer to an object than a primary mirror, then either the outer diameter of a lens barrel must be increased or a projected portion must be provided on an outer periphery of a lens barrel, which is not preferable in any case.